Funding Information

Abstract/Summary

OBJECTIVES: Microorganisms and geochemical processes lead to generation of the smallest soil particles. This project will examine the form, distribution, and abundance of nanoparticles, and the extent to which the transport of these particles leads to mobility of elements and impacts the chemistry of weathered rock. We predict that both chemical weathering and microbial activity (biomineralization, often incidental to the main metabolic activity of the cells) generate particles that are < 10 nm in diameter. Once the porosity and permeability of the weathered rock increase due to dissolution, substantial transport of insoluble, nanocrystalline solids should occur. This may lead to patterns of element distribution in the weathering profile and soil that are not anticipated if “insoluble” is equated with “immobile”. Understanding of physical transport of “immobile” elements (such as titanium and zirconium) is critical to development of models for soil formation and landscape development that often assume that certain elements are immobile.

APPROACH: The approach to study of nanoparticles during weathering and soil formation will involve field-based sample collection, detailed characterization of samples, experimental work, and modeling. Samples will be collected from a granite weathering profile for characterization of the mineralogy and bulk composition. The bulk chemistry of samples ranging from fresh granite to highly weathered granite and soil is needed so that patterns of element loss, correlated with total loss of mass (density change) can be established. Weathered minerals will be characterized via high-resolution electron microscopic methods. Fluids will be sampled for characterization of the finest particle fraction by high-resolution transmission electron microscopy. Cells will be localized for characterization of associated biominerals. Direct evidence for the pathways for redistribution of specific “immobile” elements during weathering will be established in order to determine the extent to which transport of nanoparticles (as well as colloids) occurs. Modeling will utilize existing cosmogenic isotope-based dates for landscape lowering and chemical and physical data in order to determine the absolute rates at which elements are lost from the landscape during elevation loss, and the degree to which this is accomplished via nanoparticle transport. Experimental studies of nanoparticle transport using synthetic nanocrystalline TiO2 will be conducted in subsequent years.

PROGRESS: 2006/01 TO 2006/12
Phosphate mineral dissolution provides phosphorous to the biosphere. At soil-relevant pH, P release from soil iron- and lanthanide-phosphate minerals requires metal-complexing ligands that increase mineral solubility and dissolution rates. The rate of oxalate-promoted dissolution is strongly pH dependent, while DFO-B promoted dissolution is weakly- or non-pH dependent. Studies of iron and lanthanide phosphate mineral dissolution were extended to explore the role of organic ligands in fungally-mediated apatite dissolution in California grassland soils that have been subjected to 6 years of rainfall manipulations. Active fungal isolates were screened for their ability to dissolve Ca5(PO4)2 (TCP). Two TCP dissolution pathways were identified in 14 active isolates: with and without acidification. All acidifying isolates were found to be Zygomycetes in the order of Mucorales. Non-acidifying isolates were all Ascomycetes, and dissolved TCP by formation of hydroxyapatite, which was sequestered into mycelial spheres. The Mucorales isolates acidify their substrate mainly though oxalic acid production when phosphate is available. Acidification varies with fungal growth rate. The Ascomycete in the family Trichocomaceae acidifies when phosphorus is limited and does not produce abundant oxalic acid. We conclude that oxalate production by Mucorales is an incidental side effect of metabolism whereas the Trichocomaceaea isolate possesses a pathway for apatite dissolution that is induced by phosphate limitation. Surface morphological evolution patterns suggest that apatite dissolution pathways depend on the composition and concentration of organic ligands. The primary focus of experimental work in the past year has been on quantitative data analysis, final data collection, manuscript writing and revision, with publication anticipated in early 2007. An important question is the link between microbial population structure and soil chemistry. The primary new effort in 2006 was development and testing of methods to monitor microbial diversity and activity in situ in Angelo soils and to correlate findings with soil chemistry. These methods make use of 16S rRNA gene microarray technology. This will be the focus on ongoing AES research.

IMPACT: 2006/01 TO 2006/12
We have documented relationships among soil conditions and fungal type, pH, soil organic load, and mineral dissolution rates and mechanisms. Results linking rainfall patterns and microbial community structure provide a basis for prediction of how climate changes may impact soil fertility. Organisms able to induce phosphate mineral dissolution may be particularly important, especially if increased activity of nitrogen-fixing microorganisms relieves the ecosystem nitrogen limitation. Measured rates of dissolution provide quantitative constraints that can be used to evaluate the role of fungi in the phosphorus cycle in soils.